Power steering systems have become standard equipment on all large Class 8 over the highway trucks and tractors. In that, depending on how it is equipped, a given cab, power train and chassis may be either utilized as an over the highway truck or a highway tractor, we will use the word "truck" here to designate such a vehicle recognizing that strictly speaking the vehicle may be equipped to be either a truck or a tractor.
For many years over heating of Class 8 truck power steering systems has been a problem, especially with construction and specialty vehicles, such as cement mixers and dump trucks. This over heating causes seals and other components to fail prematurely as well as causing power steering oil breakdowns.
The steering fluid high temperature problem is most evident when a truck is operated at relatively high engine speed with little or no steering activity. The problem is most prevalent in construction and specialty vehicles because their engines often operate at high speed idles for power take-off operations and slow speed steering maneuvers such as occur as a vehicle traverses a construction site. Under high engine idle conditions the steering fluid is circulated at high flow rates from a reservoir through a pump and return which leads to so called "flow work" as pressure is dropped through the system. In addition, there is a high recirculation rate of fluid through the pump which causes further fluid heat up. This further heat up occurs because the pump, being mounted on the engine block, is at engine temperature resulting in heating of the fluid.
With modern stress on the provision of aerodynamically efficient trucks and the use of larger, higher horse power engines, under hood temperatures of all trucks including highway tractors, have increased. The increased under the hood temperatures have exacerbated the problem of over heated power steering systems so that, even in highway tractors, steering oil temperature limits of 230.degree. F. and steering gear temperature limits of 240.degree. F. are all too frequently exceeded.
A further factor which has contributed to the excessive steering gear temperature problems is that the fluid pressures utilized in steering systems have been increasing and it is anticipated these pressures will be increased even further. An increase in the pressure of steering gear fluid results in increased heat load and translates to higher oil and gear temperatures.
Another factor contributing to excessive steering system temperature is that the components of the system themselves retain heat and few, if any, attempts have been made toward improving heat dissipation from these components.
In the past vehicle manufacturers have universally resorted to the provision of either large oil reservoirs or air to oil coolers, or both, for cooling of steering gear oils. When power steering overheating problems are experienced by truck operators, air to oil coolers are the standard "fix". Indeed, air to oil coolers of at least as large as 3600 Btu per hour capacity have been used as solutions for power steering oil over heating problems.
While the addition of such coolers has in many instances solved the problem of oil over heating, there are nonetheless very distinct drawbacks to this solution. One obvious drawback is the cost of providing such an air to oil cooler. Another drawback is not only does such a cooler add complexity to a system, but finding a space large enough to permit mounting of such a cooler at a location where there is sufficient air flow for it to function properly can be a problem. Typically such an air to oil cooler is positioned in front of the vehicle's coolant radiator and when so positioned it decreases the efficiency of the coolant radiator to the point where the radiator, at least potentially, is inadequate to provide appropriate cooling of the engine's coolant.